Split a stack into 2 stacks

Hey,

I just got 4 new N1524 switches that I'm looking to put into two stacks of two, with a lag between them (data centre interconnects).

I initially put all 4 in a single stack to simplify the firmware upgrade, which is done. I then restarted them in 2 distinct stacks, top two connected together and bottom two connected together. I was expecting them to get stack IDs 1 and 2 in each stack, but they are still 1, 2, 3 & 4!

They still function as 2 distinct stacks, but how do I change the IDs?